The invention relates to temperature compensated crystal oscillators.
Crystal oscillators (XO) are widely used in electronics as highly stable and accurate frequency sources. In a voltage controlled crystal oscillator (VCXO) the nominal oscillation frequency is adjustable in response to a voltage control input. The frequency accuracy of the crystal oscillators is affected by many variables, some of which are temperature, aging, drive level, retrace and vibration. As illustrated in FIG. 1, normal quartz crystal has quite large temperature variations, which depend on the cutting angle of the crystal.
At many applications the requirements for the maximum temperature variations are much tighter than this variation, and therefore various temperature compensation methods have been developed. One way to achieve frequency stability is to thermally isolate the crystal and oscillator circuitry from ambient temperatures excursions. In an oven controlled crystal oscillator (OCXO) the crystal and other temperature sensitive components are in a stable oven (a small usually metallic, insulated enclosure) provided with a heating element and a control mechanism to regulate the amount of heat applied thereby maintaining a constant elevated temperature. However, the OCXOs have disadvantages, such as the space required for the oven.
A temperature compensated crystal oscillator (TCXO) and a voltage controlled temperature compensated crystal oscillator (VCTCXO) typically contain a temperature compensation circuit to sense the ambient temperature and control the crystal frequency in order to prevent the frequency drift over the temperature range.
As illustrated in FIG. 1, normal quartz crystal has quite large temperature variations, which depend on the cutting angle of the crystal. The problem is that present manufacturing methods of the TCXO modules require temperature variations for accurate temperature compensation result. Temperature variations requires expensive ovens, which increases the manufacturing time and costs of the TCXO modules. In the TCXO modules the temperature compensation function is implemented with an integrated circuit or discrete components, which typically have quite large manufacturing tolerances. This means that the TCXO module has to be measured at several temperatures to get the correct settings for the perfect compensation result.